Poor Mr Selfridge has been in the wars, and not just the retail ones by the look of him. Sitting between me and the actor Mr Jeremy Piven, a normally sprightly 48, is his cut and bruised right foot that is propped on a chair, the swollen big toe the colour, in parts, of a blueberry sorbet. This is the result of a scooter accident, a mishap on which he is as tight-lipped as the story lines for the second season of Mr Selfridge which he is now shooting, shrugging off the pain but sighing with obvious relief to take a rest. Health has been high on his personal agenda of late: his is still suffering the ill effects of the mercury poisoning that forced his controversial withdrawal from a Broadway production of Mr David Mamet's Speed-the-Plow in 2008, an episode which has left its own tender scars.

Messrs John Cusack and Piven in the 2001 romantic comedy Serendipity

No matter. It is a heavenly day near Regent's Park and Mr Piven loves London: he spent a chunk of last year shooting the first series of Mr Selfridge and living in Belsize Park during the Olympics, enjoying the theatre, the royals (he met Prince Harry, a fan, at polo) and the sartorial freedom appreciated by a man who rejoices in his splendid green Valentino trainers. "I feel comfortable here because men are free to experiment," he says with a shrug. "In the States they assume you're being pretentious or trying to draw attention to yourself when you are just having fun with clothes."

Today he is also so luxuriantly thatched and bearded for his role, so well covered, it looks as if his handsome face might lose the fight against encroaching whiskers even as we talk, but somehow it suits his thoughtful, slightly brooding, presence. With his big dark-rimmed glasses, slow, deliberate delivery and a voice so quiet you strain at times to catch his words, he is really not what you were expecting.

His two famous roles, after all, have been flash, brash US mavericks: the ebullient Harry Selfridge and the volcanic Hollywood agent Ari Gold - "hug it out, bitch", would be his standard invitation to a brotherly embrace - in eight seasons of HBO's series Entourage, for which he won three Emmys and a Golden Globe. Are those larger-than-life characters his favourite professional territory? The question, alas, is not to his liking. "It is not my thing at all, actually," he frowns, for if Mr Selfridge's genius was in nurturing his relations with the press, Mr Piven is rather wary of us. "As you can see I am on a totally different wavelength to these characters. It's people like you that stop me from getting different types of roles in Hollywood. You assume that because I am able to tap into the authenticity of a character, that is my sweet spot. The reality is that I couldn't be further from Ari Gold. He is a Hollywood shark whose focus is money. I'm a stage actor from Chicago. Jeremy Piven would bore Ari Gold to tears. I have done 20 years of yoga with meditation. I'm viciously boring. But once you play a role that has massive speeches and your characters are verbal stunt pilots, then you're suddenly that go-to guy. The irony is that the things closest to me I haven't even played yet." Personally, he would be happiest embodying strong, silent, physical types (as well as yoga, he trains and boxes) and he credits ITV with the vision to cast him as romantic Harry after spending so long as the rage-a-holic Ari. "To be honest, had this role come along in the States I don't know if I'd have been offered it."

The Harry of the second series has been propelled forward five years to 1914 (the show's screening will coincide with the centenary of WWI) and is less of a PT Barnum showman, increasingly enslaved by his demons - gambling, chasing showgirls, frittering his fortune as his beloved wife Rose slips further away from him.

This is a dream role that the actor, an avid Downton Abbey fan, almost willed into being: and one he deserved, having worked and waited so long for his moment of fame. The son of respected actors who ran the Piven Theatre Workshop in the Chicago suburb of Evanston, he "crawled on stage with his family at eight" and has worked ever since. His first role in Chekhov's The Seagull was shared with fellow student and friend Mr John Cusack, a talent nurtured by Mr Piven's parents. After university he joined The Larry Sanders Show; in his twenties he played a studio executive in another behind-the scenes Hollywood story, Mr Robert Altman's The Player, allowed by the great director to improvise in a shot which wasn't intended to be cut, a show of trust that still moves him.

As a student he had completed an exchange programme at the National Theatre and asked its casting director if a US actor could ever work here. "She said, 'Oh, my dear boy, you have to be a star to work here.' I remember wondering what it would take to be a star and if that would ever even be possible." Instead, he worked nonstop in film; by the time his break-out role as Ari Gold arrived he had made more than 40 movies and when he won a Fresh Face of the Year award, he was 37 years old. "The first thing I said was 'there's nothing fresh about this face'. If you stick with it long enough you may fall into some luck and that's what happened to me... I just stuck
it out."

For many of those years he was frustrated playing "the schlumpy or acerbic best friend" while knowing he was capable of much more. In 29 years he never took a break until illness dictated it, a fear of unemployment for which he admits his personal relationships paid a price - he is still single despite tabloid twitterings about various dates and assignations - and his health suffered.

Ari Gold is a Hollywood shark whose focus is money. I'm a stage actor from Chicago. Jeremy Piven would bore Ari to tears

Long-term exhaustion no doubt contributed to the Broadway crisis, when he was hospitalised with mercury poisoning so severe that his doctor enquired if he had been "sucking on thermometers", a line refined by Mr Mamet (once a great friend of Mr Piven's late father), who joked that Mr Piven was taking a break to "pursue a career as a thermometer" which the actor, to his credit, defends as a funny line rather than a gentle swipe.

The problem, he tells me, deadpan, was having eaten fish twice a day for 20 years; these days he can't look a tuna fish in the eye and is still intravenously chelating organic compounds to detoxify his system. The Piven family motto insisted, of course, that the show must go on but when the doctors discovered how his father died (from pneumonia just after he had finished King Lear), he was told that going back on stage put him in imminent danger of a heart attack. "I had levels of mercury they hadn't seen in a person," he sighs, "but that doesn't get into print. Much juicier that I am perceived as a coward." Forced to appear before an Equity committee after the producers filed a grievance against him, he was totally vindicated, but badly bruised. "They knew from the first week that I was in trouble. The whole thing is a farce... but this too will pass. As Martin Luther King said, 'No lie can last forever'. I'm still here. If I was some unprofessional cop-out, don't you think we would have heard about it by now?"

Mr Piven in Mr Guy Ritchie's 2008 crime film RocknRolla

Meanwhile the work rolls in. When we met he had just been signed for Mr Doug Liman's sci-fi alien adventure Edge of Tomorrow starring Mr Tom Cruise and Ms Emily Blunt; the sequel Sin City: A Dame to Kill For with Messrs Mickey Rourke and Bruce Willis (Mr Piven's character is a corrupt cop investigating Ms Eva Green's "dame who is up to no good") is due for release next year; while the Entourage movie is hovering between a sure thing and possibility. Harry Selfridge's roller-coaster life provides enough material to furnish many more series, but their putative star believes that, despite the 8.5 million ratings garnered by the first series, the Brits have more class than the US networks with their magic number targets and syndication aspirations, and will leave us wanting more.

Mr Piven really thinks of himself a stage actor, his thorough training the reason he can nail those high-intensity characters: he studied commedia dell'arte under Mr Tim Robbins, learning how to ramp up the energy of his performances to breaking point. "I saw Tim after a few seasons on Entourage and he looked at me and started laughing because he knew where Ari had come from." Here in the home of world-class, badly paid classical actors, he feels like "a freak in a freak's kingdom", wondering if he could create an Iago as good as Mr Rory Kinnear's with which he was enraptured, if he will get the chance to convey the repressed violence of Mr Stanley Kowalski in Streetcar, which are the telling stage dreams of the Hollywood veteran.

Mr Piven is a little over sensitive, perhaps, but one warms to his honesty, his refusal to mount a phony charm offensive and, mostly, his gratitude for not just his latterly starry life, but for every job he was ever given. He has never envied the more glamorous, earlier-peaking careers of friends such as Mr Cusack, whose success he describes as a beautiful thing, impossible to resent. "My parents were proud of their students, friends, fellow actors," he concludes wisely. "It rubbed off on me. It 's a good way to live. Believe me, the other way leads to misery."